Method and processing arrangement for joint processing of uplink data

ABSTRACT

The invention relates to a method for joint processing of uplink data (g(n), s(n)) transmitted from at least one user equipment (UE 1 , UE 2 ) to a plurality of coordinated reception points (BS A , BS B ) of a wireless communication system ( 1 ), the method comprising: estimating and preferably compensating for individual propagation delays of the uplink data (g(n), s(n)) transmitted from one of the user equipments (UE 1 , UE 2 ) to the coordinated reception points (BS A , BS B ), and compensating a timing difference between a propagation delay of a coordinated reception point (BS A , BS B ) which serves the user equipment (UE 1 , UE 2 ) and at least one propagation delay of at least one coordinated reception point (BS B , BS A ) which does not serve the user equipment (UE 1 , UE 2 ) for performing the joint processing of the uplink data (g(n), s(n)), wherein the step of compensating the timing difference comprises modifying a channel matrix (H) associated with uplink channels from the at least one user equipment (UE 1 , UE 2 ) to the coordinated reception points (BS A , BS B ). The invention also relates to a processing arrangement (BS A , BS B   , 2 ) adapted for performing the method.

FIELD OF THE INVENTION

The invention relates to the field of signal processing in wirelesscommunication systems, and, in particular, to joint processing of uplinkdata transmitted from user equipments to coordinated reception points inwireless communication systems.

BACKGROUND

This section introduces aspects that may be helpful in facilitating abetter understanding of the invention. Accordingly, the statements ofthis section are to be read in this light and are not to be understoodas admissions about what is in the prior art or what is not in the priorart.

When performing joint processing (JP) of uplink coordinated multi-pointtrans-missions (so-called COMP), one has to deal with multi-pathpropagations leading to different timing/propagation delays to thedifferent reception points (e.g. base stations) of a COMP coordinationcluster. If the difference of the timing offsets due to the differentpropagation delays exceeds the length reserved for the cyclic prefix inan Orthogonal Frequency Division Multiplex, OFDM, transmission scheme,the joint processing of the uplink signals of different COMP receptionpoints (for example located in different base stations (BS), such asenhanced NodeBs “eNBs”) in a single receiver is no longer possible.

For this reason 3GPP has defined two different cyclic prefixes (CP), anormal and an extended one. To allow for joint processing, the length ofthe extended cyclic prefix can be configured to allow for acorresponding maximum difference of the timing offsets (delay spread) ofthe uplink radio signals received in the different reception points.However, using such an extended cyclic prefix has the disadvantage ofgenerating an additional overhead, which reduces the achievable overalluplink throughput. With the normal cyclic prefix, 14 OFDM symbols can beused for the transmission within one sub-frame of the duration of 1 ms.Using the extended cyclic prefix allows for only 12 OFDM symbols persub-frame. Using the extended cyclic prefix instead of the conventionalcyclic prefix thus results in a throughput reduction that amounts toabout 20 percent.

SUMMARY OF THE INVENTION

One aspect of the invention relates to a method for joint processing ofuplink data transmitted from at least one user equipment to a pluralityof coordinated reception points of a wireless communication system, themethod comprising: estimating and preferably compensating for individualpropagation delays of the uplink data transmitted from one of the userequipments to the coordinated reception points, and compensating for atiming difference between a propagation delay of a coordinated receptionpoint which serves the user equipment and at least one propagation delayof at least one coordinated reception point which does not serve theuser equipment for performing the joint processing of the uplink data,wherein the step of compensating the timing difference comprisesmodifying a channel matrix associated with uplink channels from the atleast one user equipment to the coordinated reception points.

Typically, each user equipment is served with downlink data from onlyone so-called “serving” reception point, a timing advance being providedfor aligning the timing of the user equipment to its serving receptionpoint (in the following also referred to as a cell). Thus, thepropagation delay from a user equipment to its serving coordinatedreception point may be compensated by adjusting, in the time domain, theposition of a processing window for performing atime-to-frequency-domain transformation, in particular aFast-Fourier-Transformation, FFT, on symbols of the uplink data. Adedicated timing delay/offset estimation within the layer1 processing ofthe serving cell may be used for this purpose. However, when multipleuser equipments have to be detected on the same multiple access(time/frequency/code) resource over several cells bymultiple-input-multiple-output (MIMO) receiver strategies/multi-userdetection, the problem occurs that the timing of the cells may bedifferent.

For overcoming this problem, the inventors propose to perform a timingoffset estimation also in the non-serving cells of the coordinationcluster in order to detect the correct timing of the sub-frames and OFDMsymbols per user equipment. Thus, the difference in the timingoffsets/propagation delays between different cells can be taken intoaccount and can be used to modify the channel matrix (per sub-carrier,see below) associated with the uplink transmissions for taking thetiming difference into account. In such a way, joint processing may beperformed even if the difference in the propagation delays from one userequipment to different reception points exceeds the length of the cyclicprefix being used in classical OFDMA-based systems. As this solutioncompensates the propagation delay differences of COMP cells, a type ofcyclic prefix can be used that is appropriate for the multi-pathpropagation conditions, disregarding the timing offset to thenon-serving COMP reception points. Hence, there is no need to increasethe overhead when applying the COMP reception scheme.

In one variant, the step of modifying the channel matrix comprisesperforming a phase-shift on channel coefficients of the channel matrixassociated with non-serving coordinated reception points. By multiplyingthe phase into the MIMO channel matrix of the composite COMP MIMOchannel, a modified MIMO channel matrix may be generated whichcompensates for the difference of the timing delays, thus allowing jointprocessing of the uplink data.

In an improvement of this variant, the step of modifying the channelmatrix further comprises modifying an amplitude of the channelcoefficients of the channel matrix associated with the non-servingcoordinated reception points, preferably by multiplying the channelcoefficients with a real number smaller than one. In particular when thedifference between the propagation delays of different reception pointsis large, the size of the processing window for performing atime-to-frequency domain transformation (FFT) on the user data from thenon-serving reception points may be considerably reduced. The influenceof the reduced size of the processing window may be taken into accountby multiplying the corresponding channel coefficients of the channelmatrix with a factor smaller than one, the factor depending on the sizereduction of the FFT window.

In a further variant, the uplink transmissions from the at least oneuser equipment to the coordinated reception points are performed in anorthogonal frequency division multiplex, OFDM, transmission scheme, anindividual channel matrix being preferably defined for each sub-carrierof the OFDM transmission scheme. When using such a transmission scheme,typically, an individual MIMO channel matrix is defined for each of thesub-carriers or for a group of sub-carriers used for the uplinktransmission of the same user equipment.

In one improvement the method further comprises: performing a jointcalculation of receiver antenna weights for the plurality of userequipments, the calculation being based on the modified cannel matrix.The modified MIMO channel matrix allows using a single receive combiningset of antenna weights for all the antennas of the reception points,i.e. for the whole COMP cluster. It will be understood that severaldifferent receiver types may be used for performing the jointprocessing, for example receivers of the minimum mean-square error,MMSE, and/or successive interference cancellation, SIC, type.

In a further variant, the uplink data is transferred from thecoordinated reception points to a centralized unit for performing thejoint processing, the uplink data being preferably transferred in theform of time domain or of frequency domain in-phase and quadrature-phase(IQ)-samples, in particular together with information about thepropagation delay from each user equipment. In case of frequency domainIQ-samples, also the channel coefficients have to be exchanged betweenthe base stations. When each user on its individual time-base (creatingone FFT per user equipment and performing COMP in a non-serving cellwith individually matched timing) is detected, the frequency domain IQsamples or soft bits can be transported over the backhaul to a centralpoint (usually the base station of the serving cell) for performing thejoint processing of the coordinated multipoint reception. It will beunderstood that when transferring the uplink data as time domain IQsamples, the information about the timing offset is not required but maynevertheless be present, whereas when transferring frequency domain IQsamples, typically additional information about the propagation delayhas to be transferred to the centralized unit. Of course, the uplinkdata may also be transferred to the centralized unit in another format,e.g. as soft bits or as decoded data.

In a further variant, the joint processing comprises: performing a jointtime-to-frequency domain transformation, in particular aFast-Fourier-Transformation, FFT, on time domain samples of the uplinkdata of a plurality of user equipments having similar propagation delaysto a common non-serving reception point. When two or more userequipments are present which have a similar distance from the samereception point, a joint FFT may be performed for transforming timedomain samples of the uplink data to frequency domain samples, thusreducing the overall number of FFTs being required for this purpose, andconsequently also the complexity of the calculations. It will beunderstood that for this purpose, typically the user equipments will begrouped into different sets, each having a similar propagation delay.

A further variant comprises choosing the repetition rate for estimatingthe propagation delay in dependence of the speed of movement of the userequipment. In particular, the rate for updating of the timing offsetestimation (and optionally the grouping as described above) may decreasewith decreasing mobility of the (mobile) user equipment, i.e. updateshave to be performed less often for slowly moving mobile stations thanfor rapidly moving mobile users. One skilled in the art will appreciatethat different types of user equipment with different mobilitycharacteristics may be used in the wireless network, such as mobilephones, PDAs, Laptops, or also stationary equipment.

In one improvement, the joint processing comprises: performing aFast-Fourier-Transformation, FFT, on a number of time domain samples ofthe uplink data in a processing window, the method preferably furthercomprising: shifting the processing window by subtracting a discreteFourier transform, DFT, contribution for each sub-carrier of one or moretime domain samples at one end of the processing window and adding adiscrete Fourier transform, DFT, contribution for each sub-carrier of acorresponding number of time domain samples at the other end of theprocessing window. One skilled in the art will readily recognize that inthe present case, the “processing window” is defined in the frequencydomain for a parallel processing of a number of samples corresponding toadjacent sub-carriers of the OFDMA signal.

In the way described above, a further reduction of the number ofnecessary FFTs for an OFDMA transmission scheme can be performed, asfollows: For processing a new set of samples having a FFT window whichis just shifted (in the frequency domain) by one or by several sampleswith respect to a previously processed set of samples, instead ofprocessing a new FFT for the new set of samples, it is proposed to addthe DFT contribution per sub-carrier of the one or more new samplevalues at one end of the FFT window and to subtract the first samplevalue(s) of the previous FFT window. This concept can also be combinedwith the concepts described above, e.g. first grouping the userequipments according to their propagation delays, doing a FFT for eachof the groups and shifting the FFT window in the above-described manneruntil all the user equipments of a particular group are covered, thuspossibly requiring to perform only one FFT for each group of userequipments.

One skilled in the art will appreciate that the method as describedabove may also be realized in the form of a computer program productwhich may be implemented in a suitable software or hardware, inparticular a field programmable gate array (FPGA), anapplication-specific integrated circuit (ASIC), or a digital signalprocessor (DSP). Of course, the computer program product may alsocomprise a plurality of separate parts which may be arranged atdifferent locations. It will be understood that the method may inparticular be implemented in a suitable software or hardware of areceiver for performing the joint processing, the latter possibly notperforming the first step of estimating the propagation delays, as itmay receive the information about the estimation from another physicalentity together with samples of downlink data.

A further aspect of the invention is implemented in a processingarrangement for joint processing of uplink data transmitted from atleast one user equipment to a plurality of coordinated reception pointsof a wireless communication system, the processing arrangement beingadapted to estimate and preferably to compensate for individualpropagation delays of the uplink data transmitted from one of the userequipments to the coordinated reception points, the processingarrangement being further adapted to compensate for a timing differencebetween a propagation delay of a coordinated reception point whichserves the user equipment and a propagation delay of at least onecoordinated reception point which does not serve the user equipment forperforming the joint processing of the uplink data, the processingarrangement being adapted to compensate for the timing difference bymodifying a channel matrix associated with uplink channels from the atleast one user equipment to the coordinated reception points.

The processing arrangement as described above allows for performing COMPusing joint detection without having to increase the length of thecyclic prefix, thus increasing the throughput. It will be understoodthat in the most simple case, the processing arrangement may beimplemented as a receiver for performing the joint processing. However,it may also be possible that the estimation and compensation of thepropagation delay to the serving reception point may be performed at adifferent location as the compensation of the timing difference to thenon-serving cells.

In one embodiment, the processing arrangement is adapted forcompensating the timing difference adapted for performing a phase-shifton channel coefficients of the channel matrix associated withnon-serving coordinated reception points. The channel coefficients whichare related to the non-serving reception points typically correspond tothe non-diagonal elements of the MIMO channel matrix, whereas thediagonal elements of the MIMO channel matrix represent channels betweenthe user equipment and a respective serving reception point.

In one improvement, the processing arrangement is further adapted formodifying an amplitude of the channel coefficients of the channel matrixassociated with the non-serving coordinated reception points, preferablyby multiplying the channel coefficients by a real number smaller thanone. As described above, in this way, the influence of a size reductionof a processing window for performing a FFT on the non-serving receptionpoints may be taken into account.

In one improvement, the processing arrangement is adapted to perform ajoint calculation of receiver weights for the plurality of userequipments, the calculation being based on the modified channel matrix.The receiver weight matrix may be derived from the modified channelmatrix by performing matrix calculations, as detailed further below.

In a further embodiment, the uplink data is transmitted from the atleast one user equipment to the coordinated reception points in anorthogonal frequency division multiplex, OFDM, transmission scheme, andthe processing arrangement is preferably adapted to generate anindividual channel matrix for each sub-carrier of the OFDM transmissionscheme. In this way, the transmission properties for each sub-carrier ofthe frequency band of the ODFMA transmission scheme may be optimizedindividually.

A further aspect is implemented in a base station comprising aprocessing arrangement as described above. The base station may comprisea central unit comprising a receiving device which is adapted forperforming the joint processing of the uplink data. In this case, thereception points may e.g. be implemented as remote radio heads (RRHs)being arranged remotely from the central unit and serving differentsectors of the base station, or the base station may be the servingreception point, the uplink data of further base stations, forming thenon-serving coordinated reception points, being transferred to theserving base station e.g. via a backhaul network, the estimation of thepropagation delays of the non-serving reception points being performedin the serving base station.

Yet another aspect is implemented in a wireless communication networkcomprising at least one processing arrangement as described above. Inthis case, the processing arrangement may comprise several physicalentities which may be located at different positions within the network.For example, the reception points may correspond to base stations whichare connected via a common backhaul, e.g. packet-based (IP) network, thecentral unit being located in the backhaul network. It may also bepossible to transfer uplink data from the non-serving reception pointsbeing located e.g. at different base stations, to the serving receptionpoint (serving base station), the serving base station then performingthe joint processing for the user equipment. It will be understood thattypically, the serving base station compensates for the propagationdelay e.g. in the way described above, and that the propagation delay ofthe other base stations may be taken into account by an appropriatemodification of the channel matrix.

Further features and advantages are stated in the following descriptionof exemplary embodiments, with reference to the figures of the drawing,which shows significant details, and are defined by the claims. Theindividual features can be implemented individually by themselves, orseveral of them can be implemented in any desired combination.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments are shown in the diagrammatic drawing and areexplained in the description below. The following are shown:

FIG. 1 shows a schematic diagram of an embodiment of a wirelesscommunication network,

FIG. 2 shows a schematic diagram of uplink data from two user equipmentsreceived in one of the base stations of the network of FIG. 1, and

FIG. 3 shows a schematic diagram of the shifting of a FFT processingwindow in the frequency domain.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a wireless communication network 1 which may be incompliance with e.g. the Long-Term Evolution, LTE, standard, the WiMaxstandard, or another wireless communication standard. The wirelesscommunication network 1 has a plurality of user equipments; however, forthe sake of simplicity, only a first and second user equipment UE1, UE2are shown. The first user equipment UE1 is served by a first basestation BS_(A), and the second user equipment UE2 is served by a secondbase station BS_(B) of the network 1, respectively. The term “serving”base station refers to the provisioning of downlink data from therespective base station BS_(A), BS_(B) to the user equipment UE1, UE2,typically the serving base station being the one which is closest to theserved user equipment. Each base station BS_(A), BS_(B) defines a cell,being represented by a hexagon in FIG. 1.

The first and second user equipment UE1, UE2 transmit their respectiveuplink data g(n), s(n) to both the first and to the second base stationBS_(A), BS_(B), respectively. The base stations BS_(A), BS_(B) thus formcoordinated reception points for performing uplink coordinatedmulti-point transmission (COMP) and are operatively connected to acentralized unit 2 for performing joint processing (JP) of the uplinkdata, the centralized unit 2 being arranged at a remote location, e.g.in a (high-speed) data network. Thus, the first and second base stationsBS_(A), BS_(B) are coordinated, whereas a third base station BS_(C) ofthe network 1 does not form part of the COMP coordination cluster asdefined by the first and second base stations BS_(A), BS_(B).

One skilled in the art will appreciate that alternatively, thecoordinated reception points may be implemented as different sectors(corresponding e.g. to remote radio heads, RRHs) of the same basestation, a backhaul processing unit of the base station serving as acentralized unit in this case (the propagation delays of the uplinksignal in the sectors being different e.g. due to different multiplereflections.

It will be understood that, instead of performing the joint processingin the centralized unit 2, it is also possible to perform the jointprocessing of the uplink data originating from one user equipment in itsserving base station or cell, respectively. In this case, the data ofthe at least one non-serving base station will be transferred to theserving base station, e.g. over a cable connection, a data network, etc.as indicated by a dashed line in FIG. 1. In the latter case, the basestation BS_(A) forms a centralized unit for performing joint processingof the uplink data g(n) from the first user equipment UE1, and thesecond base station BS_(B) is used as a centralized unit for jointprocessing of the uplink data s(n) from the second user equipment UE2.

In the following, time synchronicity and frequency synchronicity of thebase stations BS_(A), BS_(B) and thus also frequency synchronicity ofthe uplink data g(n), s(n) will be assumed. Further, it will be assumedthat in each base station BS_(A), BS_(B), the uplink data g(n), s(n)will be received in an individual time base which is aligned to thepropagation delay of the user equipment UE1, UE2 served by therespective base station BS_(A), BS_(B).

For the first base station BS_(A), the received signal r_(A)(t) isrepresented in FIG. 2, showing in the upper part OFDM symbols u₀(n),u₁(n), . . . of the uplink data g(n) from the first user equipment UE1which is served by the first base station BS_(A). The uplink data g(n),resp., the OFDM symbols u₀(n), u₁(n) are time-aligned to respective FFTprocessing window W1, covering only that part of the OFDM symbols u₀(n),u₁(n) which does not contain the cyclic prefix. Thus, the signal g(n)received from the served user equipment UE1 is in its optimum positionfor decoding. An analogous time alignment is also performed for theuplink data s(n) of the second user equipment UE2 which is received inthe second base station BS_(B) (the respective signal not being shownfor the sake of simplicity).

The lower part of FIG. 2 shows the OFDM symbols v⁻¹(n), v₀(n), . . . ofthe uplink data s(n) received from the second user equipment UE2 in thefirst base station BS_(A). As can be seen from FIG. 2, the position of aFFT processing window W2 for processing the uplink data s(n) is shiftedby a timing difference τ=t₁−t₂, the latter being due to the differentpropagation delays from the first and second user equipment UE1, UE2 tothe first base station BS_(A). In a similar way, the uplink data g(n) ofthe first user equipment UE1 is shifted by a timing difference withrespect to the optimum position of the FFT window. Consequently, whenperforming joint processing of uplink data g(n), s(n) received in acommon FFT window which is time-aligned to the serving base stationBS_(A), BS_(B), the influence of the time shift of the signal g(n), s(n)received in the non-serving base station BS_(B), BS_(A) has to be takeninto account.

In the following, with reference to FIG. 2, the steps will be given tocalculate the influence of the signal s(n) in OFDM symbol s₀(n) or ofthe respective sub-carrier complex amplitude S₀(f=f_(k)) at sub-carrierfrequency f_(k) on the FFT window W1 if no time shifts of the receivesignals r_(A)(t) at base station BS_(A) (or r_(B)(t) at base stationBS_(B)) are performed. For this purpose, the signal s(n) received fromUE2 depicted in the lower part of FIG. 2 will be analyzed.

If one looks at the signal s(n) in a time-base starting at t₂, thesignal is described by

s ₀ ^(CP)(n)*h _(2A)(n)=v ₀(n),

where s₀ ^(CP)(n) is the transmit signal from the first user equipmentUE1 including a cyclic prefix. In FFT window W2 the result from the FFTwould be

S ₀(ƒ)·H _(2A)(ƒ).

However, when performing a FFT on the signal s(n) using the FFT windowW1, the window function w(t) has to be taken into account. First, thereis the problem that not all sinusoids are contiguously in the windoww(t) (which introduces cross talk between the frequencies of S₀(f)).Second, even if this would be the case, still because of the timewindowing by w(t), in the frequency domain there occurs a convolutionwith a window function W(f). If one ignores the discontinuity in thetime-base starting at t₂, the windowed result is:

${{v_{0}(n)} \approx {\left\lbrack {{s_{0}(n)}\overset{cycl}{\overset{}{*}}{h_{2A}(n)}} \right\rbrack \cdot {w_{\tau}(n)}}},{and}$V₀(f) ≈ [S₀(f)H_(2A)(f)] * W_(τ)(f)

Then, the result from (that in FFT window W1 is

V ₀(ƒ)≈{[S ₀(ƒ)H _(2A)(ƒ)]*W _(τ)(ƒ)}e ^(−j2πƒτ).

Thus, for sub-carrier frequency f_(k), i.e. the value V₀(f_(k)), notonly the transmitted sub-carrier amplitude S₀(f_(k)), but alsoS₀(f_(k+1)) and S₀(f_(k+1)) are relevant (cross-talk). So the decouplingthat allowed that one sub-carrier frequency can be detected with alinear equation system in that sub-carrier frequency f_(k) only, is nolonger valid. Further, there is an additional influence from theprevious OFDM symbol, that is from s⁻¹(n), such that again, there is acoupling in time-direction between the symbols.

If one ignores also this coupling and models the previous OFDM symbolpart as noise, cross-talk over neighboring sub-carriers is stillpresent. If one also wants to ignore this frequency coupling, one hasfor the resulting coefficients in FFT window W1:

[S ₀(ƒ_(k))H _(2A)(ƒ_(k))]·W _(τ)(ƒ=0)e ^(−j2πƒ) ^(k) ^(τ).

That is, there is not only a phase factor but due to the windowing withpartial size there is also an amplitude change with a real factorW_(τ)(f=0)<1. This contribution needs to be taken into accountespecially for larger shifts. It may also influence the rank of thederived matrix that needs to be inverted, e.g. for use in a MMSEreception algorithm, which will be detailed in the following.

The uplink COMP joint processing system 1 which is exemplarilyillustrated in FIG. 1 may be regarded as a composite MIMO system with anassociated channel matrix H, having a dimension corresponding to thenumber of receiver antennas times the number of transmitter antennas,being a 2×2 matrix in the present (simplified) example with two userequipments UE1, UE2 serving as transmitters and two base stationsBS_(A), BS_(B) serving as receivers (and supposing that each of these isonly equipped with a single antenna). Thus, in the present example, thechannel matrix H is given by:

${H = \begin{pmatrix}h_{1A} & h_{2A} \\h_{1B} & h_{2B}\end{pmatrix}},$

wherein the first row vector h_(1A), h_(2A) is related to the firstuplink data g(n) transmitted from the first user equipment UE1 to thefirst and second base station BS_(A), BS_(B), respectively, and thesecond row vector h_(1B), h_(2B) is related to second uplink data s(n),being transmitted from the second user equipment UE2 to the first andsecond base stations BS_(A), BS_(B).

In the present example, the wireless communication network 1, moreprecisely the user equipments UE1, UE2 and the base stations BS_(A),BS_(B), are adapted for performing uplink transmissions in an orthogonalfrequency division multiplex, OFDM, transmission scheme, dividing thefrequency band reserved for the uplink transmission into a plurality oforthogonal sub-carriers. For such an OFDMA transmission scheme, anindividual channel matrix H may be defined for each sub-carrier (or fora group of sub-carriers).

A receiver in the centralized unit 2 will jointly process the uplinkdata g(n), s(n) from the user equipments UE1 and UE2 and may e.g. bedevised as a linear Minimum Mean Square Error, MMSE, receiver whichcalculates its weight matrix

$G = {\begin{pmatrix}g_{1A} & g_{1B} \\g_{2A} & g_{2B}\end{pmatrix} = \begin{pmatrix}g_{1}^{T} \\g_{2}^{T}\end{pmatrix}}$

as a function of the channel matrix H (see below). The first row vectorof the weight matrix G is the complex antenna weight set used to decodethe first uplink data g(n) received from the first user equipment UE1,the second row vector is the antenna weight set used to decode thesecond uplink data s(n) received from the second user equipment UE2. Theformula for the calculation of the weight matrix G in dependence of thechannel matrix H for the MMSE receiver is given as follows:

G _(MMSE)=(H ^(H) H+σ _(n) ² I)⁻¹ H ^(H),

wherein H^(H)=H*^(T) is the Hermitian conjugate of the matrix H, σ_(n)is the standard deviation of the Gaussian noise due to the transmission,an the letter I designates the identity matrix.

As described above, a timing advance for the propagation delay of theuplink data g(n) to first user equipment UE1 may be implemented in theserving base station BS_(A) by shifting the processing window W1 of thetime-to-frequency transformation (FFT) accordingly, for example by usinga timing offset estimation within the layer1 processing of the servingbase station BS_(A). Likewise, a timing advance may be performed for thesecond uplink data s(n) being received in the second base station BS_(B)serving the second user equipment UE2.

The above formula for calculating the receiver weight matrix G ishowever only valid when both user equipments UE1, UE2 are processed on acommon time basis, using a joint Fast-Fourier-Transform. However, asoutlined above, the user equipment UE1 has a different distance to thefirst and to the second base station BS_(A), BS_(B), respectively, andthe propagation delay of the uplink data g(n) from the first userequipment UE1 to the first and second base station BS_(A), BS_(B) willbe different.

Thus, when multiple user equipments shall be detected on the samemultiple access (time/frequency/code) resource over several basestations BS_(A), BS_(B) by MIMO receiver strategies/multi-userdetection, the problem occurs that the timing between the uplink signalsg(n), s(n) being received in the different base stations BS_(A), BS_(B)is completely different.

This problem may be mitigated in an OFDMA transmission system by usingan (extended) cyclic prefix that is large enough for the multi-pathpropagation plus additional propagation differences of the multi-cellCOMP system 1. However, using such an extended prefix will increase theoverhead and thus reduce the throughput of the system.

In order to avoid such an (extended) cyclic prefix, the timing for eachuser equipment UE1, UE2 to its own serving base station BS_(A), BS_(B)is controlled by the timing advance in the way described above. For theuplink signals g(n), s(n) from the user equipment UE1, UE2 which arereceived in the respective non-serving base stations BS_(B), BS_(A), aseparate timing offset/propagation delay estimation is performed andtheir FFT processing window may be shifted by a corresponding amount inorder to be aligned with the individual symbol borders of the userequipments UE1, UE2.

However, the different additional (uncontrolled) propagation delays tonon-serving base stations BS_(B), BS_(A) for the first and second userequipment UE1 and UE2 have to be taken into account. The derivationsabove now allow one to define a modified channel matrix:

$H_{mod} = \begin{pmatrix}h_{1A} & {h_{2A}{W_{\tau \; 2A}\left( {f = 0} \right)}{\exp \left( {{- j}\; 2\; \pi \; f_{k}\tau_{2\; A}} \right)}} \\{h_{1B}{W_{\tau \; 1B}\left( {f = 0} \right)}{\exp \left( {{- j}\; 2\; \pi \; f_{k}\tau_{1B}} \right)}} & h_{2B}\end{pmatrix}$

The modified channel matrix H_(mod) may then be used for calculating thecomplex weight matrix G in the centralized unit 2 (see below), theinter-symbol interference being treated as noise.

For the aggregation of the uplink data s(n), g(n) from the differentcoordinated reception points BS_(A), BS_(B) to the centralized unit 2(coordinator), several options exist: For example, the uplink data s(n),g(n) may be transferred in the form of time domain (IQ) samples to thecentralized unit 2. One skilled in the art will appreciate that IQsamples correspond to the Quadrature Phase-Shift Keying (QPSK)modulation format, but that other modulation formats may be used aswell.

The timing offset estimation may be performed e.g. in the centralizedunit 2. Alternatively, each base station may perform a timing offsetestimation for searched user equipments and a calculation of the channelcoefficients (in the UE-specific time-base) on its own, and also performthe calculation of the modified channel matrix.

Alternatively, the uplink data may be transferred as frequency domain IQsamples, in the form of soft bits or as decoded data to the centralizedunit 2. It will be understood that in any case, the uplink data UD1, UD2may be transferred to the centralized unit 2 together with informationabout the timing offset/propagation delay.

In particular, when frequency domain IQ samples are transferred, thesteps for multi-user detection may be as follows:

-   -   a) In each cell the served user equipments are time-adjusted by        TA (timing advance) commands,    -   b) for non-served user equipments, the timing is estimated to        find the UE time base. FFTs are performed for doing the channel        estimation based on demodulation reference symbols, DM RS, in        this shifted time base. The result is the set of channel        coefficients per sub-carrier for this timing, and    -   c) the frequency domain I/Q samples from FFTs of the original        time base are exchanged together with non-served UE's timing        information and channel coefficient information for these UEs,        such that the receiving base station can then calculate the        modified channel matrix.

Thus, for an optimized performance of a centralized weight processing inthe centralized unit 2, it is proposed to transfer the uplink data inthe form of time or frequency IQ samples, taking into account the timingoffset and the corresponding phase shift per sub-carrier in the updatedchannel matrix H_(mod), and processing the receiver weights accordingto:

G _(MMSE)=(H _(mod) ^(H) H _(mod)+σ_(n) ² I)⁻¹ H _(mod) ^(H),

i.e. by replacing the channel matrix H by the modified channel matrixH_(mod).

Those skilled in the art will appreciate that although the approach forcalculating the weight matrix G in dependence of the modified channelmatrix H_(mod) has been described for the example of a MMSE receiver,this approach is not limited to a specific receiver type, and may alsobe applied to other receiver types.

In the following, several ways for reducing the computational complexityof the present approach will be explained, the computational complexityin particular being reduced by reducing the number of FFTs required forperforming the time-to-frequency transformation of the time domainsamples of the uplink data.

First, a single joint FFT may be performed for a set of user equipmentswhich have roughly the same propagation delays to the non-serving basestation, resp. cell. For that purpose, the user equipments may begrouped into different sets, each set comprising only user equipmentshaving similar propagation delay.

A further approach for complexity reduction may be to update the timingoffset estimation (and optionally the grouping as described above) independence of the velocity of the user equipments: For slowly movingmobile stations, updates have to be performed less often than forhigh-speed mobile stations.

A further complexity reduction by reducing the number of necessary FFTsmay be achieved in the following way: After a FFT has been performed fora set of time domain samples, e.g. samples a₀ to a₅ being related todifferent sub-carriers in a processing window W in the frequency domain,as shown in FIG. 3, instead of performing another FFT for a processingwindow W′ being shifted by two neighbouring time domain samples a₆, a₇,it is proposed to add the DFT(discrete Fourier transform) contributionper sub-carrier of the two new sample values a₆, a₇ from the DFT sum andto subtract the first two sample values a₀, a₁ from the DFT sum, thusresulting in the processing of the samples a₂ to a₇ in the shiftedprocessing window W′.

The above approach is based on the fact that the DFT sum for thetime-to-frequency domain transformed sample â_(k) of the k-thsub-carrier of the OFDMA scheme can be written as:

${\hat{a}}_{k} = {\sum\limits_{j = 0}^{N - 1}{^{{- 2}\pi \; { \cdot \frac{jk}{N}}} \cdot a_{j}}}$

with k=0, . . . , N−1; wherein N represents the number of sub-carriersfor which the processing window is defined.

One skilled in the art will appreciate that using such an approach mayresult in performing only a few FFTs and shifting the processing windowin the above-described manner for generating the other frequency domainsamples â_(k). For instance, it may be possible to group the userequipments UE1, UE2 according to their propagation delays, to perform aFFT for each group of user equipments UE1, UE2 and to shift the FFTwindow in the way described above until all of the user equipments of aparticular group are covered.

In summary, by applying the approach described above, uplink coordinatedmulti-point transmissions may be performed without having to enlarge thecyclic prefix of the OFDMA scheme. Using the above approach thus avoidsincreasing the overhead due to such a cyclic prefix extension, thusincreasing the spectral efficiency of the COMP scheme. It will bereadily understood that the approach described above may also be used intransmission schemes being derived from the OFDMA scheme, for example inthe so-called Single-Carrier Frequency Division Multiple Access,SC-FDMA, transmission scheme, being a linear pre-coded OFDMA scheme.

The above description of the preferred embodiments has been given by wayof example. From the disclosure given, those skilled in the art will notonly understand the present invention and its attendant advantages, butwill also find apparent various changes and modifications to thestructures and methods disclosed. The applicant seeks, therefore, tocover all such changes and modifications as defined by the appendedclaims, and equivalents thereof.

1. Method for joint processing of uplink data (g(n), s(n)) transmittedfrom at least one user equipment (UE1, UE2) to a plurality ofcoordinated reception points (BS_(A), BS_(B)) of a wirelesscommunication system (1), the method comprising: estimating andpreferably compensating for individual propagation delays of the uplinkdata (g(n), s(n)) transmitted from one of the user equipments (UE1, UE2)to the coordinated reception points (BS_(A), BS_(B)), and compensatingfor a timing difference (τ) between a propagation delay of a coordinatedreception point (BS_(A), BS_(B)) which serves the user equipment (UE1,UE2) and at least one propagation delay of at least one coordinatedreception point (BS_(B), BS_(A)) which does not serve the user equipment(UE1, UE2) for performing the joint processing of the uplink data (g(n),s(n)), wherein the step of compensating the timing difference comprisesmodifying a channel matrix (H) associated with uplink channels from theat least one user equipment (UE1, UE2) to the coordinated receptionpoints (BS_(A), BS_(B)) by performing a phase-shift on channelcoefficients (h_(1B), h_(2A)) of the channel matrix (H) associated withnon-serving coordinated reception points (BS_(B), BS_(A)), wherein thestep of modifying the channel matrix (H) further comprises modifying anamplitude of the channel coefficients (h_(1B), h_(2A)) of the channelmatrix (H) associated with the non-serving coordinated reception points(BS_(B), BS_(A)),
 2. Method according to claim 1, wherein the step ofmodifying the amplitude of the channel coefficients (h_(1B), h_(2A)) ofthe channel matrix (H) is performed by multiplying the channelcoefficients (h_(1B), h_(2A)) with a real number smaller than one. 3.Method according to claim 1, wherein uplink transmissions from the atleast one user equipment (UE1, UE2) to the coordinated reception points(BS_(A), BS_(B)) are performed in an orthogonal frequency divisionmultiplex, OFDM, transmission scheme, an individual channel matrix (H)being preferably defined for each sub-carrier of the OFDM transmissionscheme.
 4. Method according to claim 1, further comprising: performing ajoint calculation of receiver antenna weights (G) for the plurality ofuser equipments (UE1, UE2), the calculation being based on the modifiedcannel matrix (H_(MOD)).
 5. Method according to claim 1, furthercomprising: compensating the propagation delay from a user equipment(UE1) to its serving coordinated reception point (BS_(A)) by adjustingthe position of a processing window (W1) for performing atime-to-frequency-domain transformation, in particular aFast-Fourier-Transformation, FFT, on symbols (u₀(n), u₁(n), . . . ) ofthe uplink data (g(n)).
 6. Method according to claim 1, wherein theuplink data (g(n), s(n)) is transferred from the coordinated receptionpoints (BS_(A), BS_(B)) to a centralized unit (2) for performing thejoint processing, the uplink data (g(n), s(n)) being preferablytransferred in the form of time domain or of frequency domain in-phaseand quadrature-phase samples (a₀ to a₇), in particular together withinformation about the propagation delay from each user equipment (UE1,UE2).
 7. Method according to claim 1, wherein the joint processingcomprises: performing a joint time-to-frequency domain transformation,in particular a Fast-Fourier-Transformation, FFT, on time domain samplesof the uplink data (g(n), s(n)) of a plurality of user equipments (UE1,UE2) having similar propagation delays to a common non-serving receptionpoint (BS_(C)).
 8. Method according to claim 1, further comprising:choosing the repetition rate for estimating the propagation delay independence of the speed of movement of the user equipment (UE1, UE2). 9.Method according to claim 1, wherein the joint processing comprises:performing a Fast-Fourier-Transformation, FFT, on a number of timedomain samples (a₀ to a₇) of the uplink data (g(n), s(n)) in aprocessing window (W), the method preferably further comprising:shifting the processing window (W) by subtracting a discrete Fouriertransform, DFT, contribution for each sub-carrier of one or more timedomain samples (a₀, a₁) at one end of the processing window (W) andadding a discrete Fourier transform, DFT, contribution for eachsub-carrier of a corresponding number of time domain samples (a₆, a₇) atthe other end of the processing window (W).
 10. Processing arrangement(BS_(A), BS_(B), 2) for joint processing of uplink data (s(n), g(n))transmitted from at least one user equipment (UE1, UE2) to a pluralityof coordinated reception points (BS_(A), BS_(B)) of a wirelesscommunication system (1), the processing arrangement being adapted toestimate and preferably to compensate individual propagation delays ofthe uplink data (s(n), g(n)) transmitted from one of the user equipments(UE1, UE2) to the coordinated reception points (BS_(A), BS_(B)), theprocessing arrangement being further adapted to compensate a timingdifference between a propagation delay of a coordinated reception point(BS_(B), BS_(A)) which serves the user equipment (UE1, UE2) and apropagation delay of at least one coordinated reception point (BS_(A),BS_(B)) which does not serve the user equipment (UE1, UE2) forperforming the joint processing of the uplink data (s(n), g(n)), theprocessing arrangement being adapted to compensate the timing differenceby modifying a channel matrix (H) associated with uplink channels fromthe at least one user equipment (UE1, UE2) to the coordinated receptionpoints (BS_(A), BS_(B)) by performing a phase-shift on channelcoefficients (h_(1B), h_(2A)) of the channel matrix (H) associated withnon-serving coordinated reception points (BS_(B), BS_(A)), theprocessing arrangement being further adapted for modifying an amplitudeof the channel coefficients (h_(1B), h_(2A)) of the channel matrix (H)associated with the non-serving coordinated reception points (BS_(B),BS_(A)).
 11. Processing arrangement according to claim 10, being adaptedfor modifying the amplitude of the channel coefficients (h_(1B), h_(2A))of the channel matrix (H) by multiplying the channel coefficients(h_(1B), h_(2A)) with a real number smaller than one.
 12. Processingarrangement according to claim 10, adapted to perform a jointcalculation of receiver weights (G) for the plurality of user equipments(UE1, UE2), the calculation being based on the modified cannel matrix(H_(MOD)).
 13. Processing arrangement according to claim 10, wherein theuplink data (s(n), g(n)) is transmitted from the at least one userequipment (UE1, UE2) to the coordinated reception points (BS_(A),BS_(B)) in an orthogonal frequency division multiplex, OFDM,transmission scheme, and wherein the processing arrangement ispreferably adapted to generate an individual channel matrix (H) for eachsub-carrier of the OFDM transmission scheme.
 14. Base station (BS_(A),BS_(B)) comprising a processing arrangement according to claim
 10. 15.Wireless Communication Network (1) comprising at least one processingarrangement (BS_(A), BS_(B), 2) according to claim 10.